1. Field of the Invention This invention relates to lock mechanisms for holding target wafers to articles of merchandise and more particularly it concerns a novel lock construction which can be manufactured easily and economically and which provides strong and reliable locking with positive release.
2. Description on the Prior Art
U.S. Pat. No. 4,590,461 shows a lock mechanism for holding a target wafer to an article of merchandise. The target wafer is a thin flat plastic housing in which an electrical circuit is embedded. The electrical circuit is designed to produce a characteristic disturbance to an electromagnetic interrogation signal when the article of merchandise is carried past a doorway or other exit path where the interrogation signal is generated. This disturbance is detected by a monitor at the doorway and the monitor in turn actuates an audio or visual alarm. The lock mechanism is releasable by means of a special magnetic tool under the control of a sales clerk or other authorized person; and upon release of the lock mechanism, the target wafer is removed from the merchandise so that the merchandise can be carried out through the doorway without actuating the alarm. The lock mechanism shown in U.S. Pat. No. 4,590,461 is known as a "ball clutch" type lock mechanism. This mechanism comprises a cone and an internally tapered ring arranged within the wafer housing and a spring which presses the cone into the tapered ring. The cone has an axial hole to accommodate the shank of a pin fastener and a pair of transverse holes which intersect the axial hole and which accommodate locking balls. When a fastener pin is projected into the axial hole it passes between the balls. The cone holds the balls in position between the pin shank and the tapered wall of the ring. Any axial force on the pin in the direction of pin removal causes the balls to become more tightly squeezed between the pin and the ring wall. The lock is released by application of a magnetic force to the cone to pull it against the force of the spring in a direction opposite the direction of pin removal. This cone movement brings the balls to a position along the ring wall where they are no longer wedged between the pin and wall; and the pin may then be easily removed.
The above described lock mechanism is very strong and secure and it operates very reliably to release the pin when a magnetic force is applied to the cone. The mechanism however is somewhat expensive to manufacture in that the cone and ring must be individually machined and a separate spring and balls must also be provided.
U.S. Pat. No. 4,722,119 describes another lock mechanism which comprises a one piece sheet metal element which is slit to form flange-like jaws that bend up and away from each other when the shank of a pin type fastener is pushed between them. Any axial force on the fastener in the direction of removal forces the flange jaws more tightly against the pin shank. However, when a magnetic force is applied to risers extending from the flange jaws, this force, according to the patent, pulls the jaws in a direction away from pin removal and forces them apart from each other and from the pin shank so that the pin may be removed.
The above described lock mechanism of U.S. Pat. No. 4,722,119 is of one piece construction; however, since the flanges which lock the pin must be flexed to release the pin, the device is either too rigid to permit reliable release or it is too flexible to provide secure locking.